The present invention relates to a switching power supply and an illumination device using the power supply. More particularly, the present invention relates to a power supply configuration for achieving precision current control.
Referring to FIG. 3, an example of a power supply as previously known in the art may be described. An inexpensive n-channel MOSFET is used as a switching element Q of a buck converter with its source electrode grounded. A pulse width modulated (PWM) signal can be supplied from a control circuit 1 to a gate electrode of the switching element Q. However, in this case it is necessary to provide a current detecting resistor R for detecting a current flowing to the light-emitting diode 3 at all times on a high-potential side. The reason is that the current detecting resistor R needs to be provided in a regenerating current path to detect a regenerating current flowing from an inductor L through a diode D when the switching element Q is turned off. Therefore, a high-side amplifier 4 for feedback is provided to detect a voltage across the current detecting resistor R. However, in this case, level shifting is necessary for transmitting a detection signal to the control circuit 1 on a low-potential side, which complicates the structure and increases costs.
Referring now to FIG. 4, a second conventional example is shown. To provide the current detecting resistor R of the buck converter on a low-potential side, a regenerating current path from the inductor L through the diode D may be provided on the low-potential side. In this case, advantageously, the control circuit 1 can directly detect the voltage across the current detecting resistor R. However, it then needs to use a relatively expensive p-channel MOSFET as the switching element Q of the buck converter or provide a high-side driver 5 for transmitting the PWM signal output from the control circuit 1 to a high-potential side.
With regards to FIG. 5, an example of operation of power supplies such as shown in FIGS. 3-4 may be described. A voltage Vr across the current detecting resistor R is graphically represented with respect to time. A gradually increasing current flows to the current detecting resistor R in an ON period T1 of the switching element Q and a gradually decreasing current flows to the current detecting resistor R in an OFF period T2 of the switching element Q. The control circuit 1 in FIG. 3 or FIG. 4 feedback controls an ON time of the PWM signal so as to make constant an average value of the current flowing to the current detecting resistor R.
With regards to either of the examples as shown in FIG. 3 or FIG. 4, a detecting circuit (high-side amplifier 4) or a driving circuit (high-side driver 5) needs to be provided on a high-potential side of a power supply voltage, which generally results in a more complex circuit structure. Furthermore, when an attempt is made to incorporate the control circuit 1 and the switching element Q together with circuits on the high-potential side into an integrated circuit, it is difficult to secure a dielectric voltage.
Referring now to the configuration of FIG. 6, by positioning the current detecting resistor R out of the regenerating current path, the switching element Q is directly driven on the low-potential side based on a current detection result on the low-potential side. With such a structure, the control circuit 1 and the switching element Q are incorporated into an integrated circuit. However, in this case, the current detecting resistor R cannot detect the whole current flowing to a light-emitting element 3 and thus, an accurate feedback control cannot be performed.
FIG. 7 shows a waveform of the current flowing to the current detecting resistor R in FIG. 6. As shown in this figure, the current flowing to the light-emitting element 3 can be detected in the ON period T1 of the switching element Q, while the current flowing to the light-emitting element 3 cannot be detected in an OFF period of the switching element Q. When the current detecting waveform in FIG. 7 is used for the feedback control, the whole current flowing to the light-emitting element 3 is not reflected, making precision current control difficult.